The FBI and the U.S. Department of Justice have acknowledged that flawed testimony from FBI scientists based on forensic evidencehas resulted in wrongful conviction in thousands of cases - including some people on death row.

An investigation launched in 2012 after it was discovered that evidence based on flawed forensic testing of hair found at crimes scenes could have led to the conviction of potentially innocent people for serious crimes such as rape and murder in the 1980s and 1990s.

This week, it was reported that of the 10 per cent of cases that have been reviewed thus far, the 'vast majority' were mistakes.

Exact science: Before DNA testing, FBI scientist's testified in cases on evidence obtained using an inexact microscopic comparison method, which means thousands of convictions must be reexamined

New Scientist reports that 136 defendents, including two who are on death row, will receive letters explaining their rights to DNA testing to prove their evidence.

A further 23 similar letters were sent out last year, and 14 of those were on death row.

The hair analysis test in question involved scientist looking at hairs recovered from crimes scenes under a microscope.

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They would compare these to hairs from the defendent to find a match - but in the days before DNA testing, it was impossible to confirm the match to the exclusion of all others, as the scientists claimed.

The Washington Post reports that the FBI stopped its review of convictions last August after the initial troubling findings but resumed this month after at the Justice Department's orders.

A report from the department's inspector general found that the FBI and Justice Department didn't move quickly enough to identify the cases handled by 13 FBI crime lab examiners whose work was found to be flawed, meaning defendants sometimes were never notified that their convictions may have been based on bad science.

It took almost five years for the FBI to identify the more than 60 death-row defendents whose cases required further examination, and during that time at least three were executed.

'I don’t know whether history is repeating itself, but clearly the [latest] report doesn’t give anyone a sense of confidence that the work of the examiners whose conduct was first publicly questioned in 1997 was reviewed as diligently and promptly as it needed to be,' Michael R. Bromwich, who was inspector general from 1994 to 1999, told the Washington Post.

Delays: So far, only 10 per cent of the cases that need to be reexamined have been looked at, but the 'vast majority' were found to have hinged on flawed science

The investigation was stopped due to 'a vigorous debate that occurred within the FBI and DOJ about the appropriate scientific standards we should apply when reviewing FBI lab examiner testimony — many years after the fact,' according to the FBI.

'Working closely with DOJ, we have resolved those issues and are moving forward with the transcript review for the remaining cases,' the FBI said.

The flawed hair and fiber comparison is just one of many methods used by FBI scientists that could prove problematic.

According to New Scientist, ballistics, fibre analysis, tyre and shoeprint comparison and tool and bite-mark analysis all use a similar inexact approach.

Experts say this could be just the tip of the iceberg and that there will be a landslide of similar cases involving state labs which used scientists trained by the FBI.

'Nobody who attended any of the FBI classes was told to over testify, that's not part of the training,' Max Houck, a former FBI hair analyst who taught some of these classes and now heads the Department of Forensic Sciences in Washington DC told New Scientist.

'However, our fear was always that we teach these people for two weeks, and they would go back to their laboratories with a certificate of completion and be told: "Great you're qualified to do this – here's your caseload."'

So far, 2,600 cases are being questioned but only 10 per cent have been reexamined in the two years since the investigation begun.

Peter Neufeld, co-director of the Innocence Project which fights to free innocent people using DNA testing, says it's rare that a person would be convicted on a single piece of evidence, but that hair matching would have carried heavy weight.

'Combined with an unreliable witness statement, it could create a perfect storm,' he told New Scientist.